Passively athermalized infrared imaging system and method of manufacturing same

ABSTRACT

In an embodiment, a method of aligning elements in a manufacturing process includes placing a middle element onto a base element, the base element forming first alignment features, the middle element forming apertures therethrough corresponding to the first alignment features. The method also includes placing second alignment features of an upper element onto the first alignment features such that the first and second alignment features cooperate, through the apertures, to align the upper element with the base element. An infrared lens assembly includes a lens formed of an infrared transmitting material that is disposed within a carrier of a base material, the lens being molded within the carrier with at least one feature that secures the lens to the carrier.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 61/434,657, filed 20 Jan. 2011, which is incorporated by referencein its entirety.

U.S. GOVERNMENT SUPPORT

The present invention was made under Phase I SBIR Contract No.IIP-1047405 from the National Science Foundation. The Government hascertain rights in this invention.

BACKGROUND

Conventional longwave infrared (“LWIR”) lenses are expensive tomanufacture and often incorporate either moving parts or nested barrelsfor athermalization. Athermalization is a larger concern for infrared(“IR”) imaging systems than for systems that image visible radiation,due to the relative increased sensitivity to temperature of common LWIRmaterials in comparison to common visible spectrum materials. FIG. 1illustrates one prior art manufacturing process 100 for producing acamera assembly. In process 100, aspheric lens elements are diamondturned individually in step 102, and a housing is fabricated in step104. The lenses are placed into a lens mount with multiple componentathermalization mechanisms in step 106. The lens mount is assembled intoa camera in step 108. The camera is actively aligned and focused usingfeedback from acquired images, and then potted into place, in step 110.The camera is calibrated and tested in step 112.

FIG. 2 illustrates, in an exploded view, one prior art lens stack 200that may be produced by steps 102 through 108 of process 100 of FIG. 1.Lenses 202 and 206 are diamond turned in step 102 of process 100.Housing 216 is fabricated in step 104. In step 106, lenses 202 and 206mount with a spacer 204 therebetween and with a first barrel 210 and asecond barrel 212 that has a high coefficient of thermal expansion.These components then mount with a threaded barrel 214 that has a lowcoefficient of thermal expansion. In step 108, threaded barrel 214screws into housing 216. Lens stack 200 may integrate with a sensor 218to form a camera.

As shown in FIG. 2, lens stack 200 includes a significant number ofassociated mounting hardware elements and interfaces between theseelements and lenses. Those skilled in the art will appreciate that theseelements generate tolerance stack-up issues (e.g., related to lensalignment, element thickness variations, tilt of the elements andmounting materials) that can make the performance of lens stack 200nonideal. Therefore, lens stack 200 is usually actively aligned andfocused in situ with a camera body in step 110 to mitigate the effectsof these issues, but the active alignment process adds further cost toprocess 100.

Another prior art method of making cameras includes molding lenses ontoone or both sides of a transparent glass substrate. Multiple wafers oflenses are then stacked on top of each other with spacer wafers betweenthe lens wafers to achieve a required spacing between the lenses. Goodimaging performance of the final lens assemblies requires precisepositioning of all of the wafers in six degrees of freedom with respectto each other: typical Cartesian x and y coordinates for centering thelens elements; z spacing between the lens elements, and rotations knownin the art as tip, tilt and theta. The required alignments are generallyperformed utilizing a mask aligner adapted from semiconductor processingequipment. Such equipment may be costly and time consuming to operate,and presents special challenges for assembly of IR optics. IR imaginginstrumentation would generally be a nonstandard and costly addition toa mask aligner, and the longer wavelengths of IR as opposed to visiblelight may make alignment thereby less precise.

SUMMARY OF THE INVENTION

In an embodiment, a passively athermalized infrared imaging systemincludes an object side meniscus lens that forms at least one asphericsurface, and an image side meniscus lens that forms two asphericsurfaces. Each of the meniscus lenses are formed of a material selectedfrom the group consisting of a chalcogenide glass, germanium, silicon,gallium arsenide, zinc selenide and glass. An optical power of the imageside meniscus lens is at least 1.6 times an optical power of the objectside meniscus lens such that an effective focus position of the imagingsystem is athermalized over a range of 0 to +40 degrees Celsius.

In an embodiment, a passively athermalized infrared imaging systemincludes an object side meniscus lens, formed of a first material thattransmits infrared radiation, that forms at least one aspheric surface;and an image side meniscus lens, formed of a second material thattransmits infrared radiation, that forms two aspheric surfaces. Theobject side and image side meniscus lenses cooperate to form a thermalimage and are concave towards the image. The first and second materialshave thermal glass constants T_(g1) and T_(g2) respectively, whereinT_(g2) is at least 1.67 times T_(g1), such that an effective focusposition of the system is athermalized over a temperature range of 0 to+40 degrees Celsius.

In an embodiment, a method of aligning elements in a manufacturingprocess includes placing a middle element onto a base element, the baseelement forming first alignment features, the middle element formingapertures therethrough corresponding to the first alignment features.The method also includes placing second alignment features of an upperelement onto the first alignment features such that the first and secondalignment features cooperate, through the apertures, to align the upperelement with the base element.

In an embodiment, a method of aligning elements in a manufacturingprocess includes placing a middle element onto a base element, the baseelement forming first alignment features, the middle element formingapertures therethrough corresponding to the first alignment features.The method also includes placing intermediate alignment elements uponthe first alignment features, and placing second alignment features ofan upper element onto the intermediate alignment features such that thefirst, intermediate, and second alignment features cooperate to alignthe upper element with the base element.

In an embodiment, a plurality of infrared lens systems includes at leastone first lens wafer comprising a first base material that is opaque toinfrared radiation, with infrared transmissive material inset intoapertures therein to form first lenses, bonded with at least one secondlens wafer having a plurality of second lenses, such that pluralities ofthe first and second lenses align to form the lens systems.

In an embodiment, a lens wafer for use in optical manufacturing includesa substrate forming apertures therein, the substrate being formed of abase material, and a plurality of lenses, each of the lenses comprisingan optical material and disposed within a respective one of theapertures.

In an embodiment, an infrared lens assembly includes a lens formed of aninfrared transmitting material that is disposed within a carrier of abase material, the lens being molded within the carrier with at leastone feature that secures the lens to the carrier.

In an embodiment, a mold set includes an upper mold and a lower mold, atleast one of the upper and lower molds having one or more features thatare configured to hold an infrared lens in an aligned position, theupper and lower molds configured to provide a cavity for molding amoldable material into one of a lens carrier and a lens wafer about theinfrared lens.

In an embodiment, an infrared lens assembly includes a lens formed of aninfrared transmitting material that is disposed within a carrier of abase material, the carrier being molded around the lens with at leastone feature that secures the lens to the carrier.

In an embodiment, an infrared lens stack includes a first lens formed ofan infrared transmitting material and disposed within a first carrier,formed of a base material, that forms at least one first alignmentfeature; and a second lens formed of an infrared transmitting materialand disposed within a second carrier, formed of a base material, thatforms at least one second alignment feature configured to cooperate withthe first alignment feature. The stack is formed by placing the firstassembly atop the second assembly so that the alignment featurescooperate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates one prior art manufacturing process for producing acamera assembly.

FIG. 2 illustrates, in an exploded view, a prior art lens stack producedby steps of the process of FIG. 1.

FIG. 3 shows a parallel LWIR manufacturing process, in an embodiment.

FIG. 4 is a perspective view showing one exemplary lens stack producedby the process of FIG. 3, in an embodiment.

FIG. 5 illustrates a wafer that forms an array of apertures where lenseswill be fabricated, in an embodiment.

FIG. 6 shows a portion of a wafer having a single aperture that iswithin recesses, in an embodiment.

FIG. 7 shows molds and moved into position for molding about theaperture so as to mold the slug into a lens, in an embodiment.

FIG. 8 shows the lens of FIG. 7 with the molds removed.

FIG. 9 is a cross-sectional view of a lens, in an embodiment.

FIG. 10 is a bottom plan view looking upward through the lens of FIG. 9.

FIG. 11 shows a mold set including upper and lower molds separated fromthe lens of FIG. 9, in an embodiment.

FIG. 12 shows the molds of FIG. 11 in contact with the lens of FIG. 9.

FIG. 13 is a cross-sectional view of a lens carrier produced by moldingan appropriate base material around the lens of FIG. 9, using the moldsof FIG. 11, in an embodiment.

FIG. 14 is a top plan view looking downward at the lens carrier of FIG.13.

FIG. 15 schematically illustrates a lens carrier produced by mountingthe lens of FIG. 9 to a lens carrier blank, in an embodiment.

FIG. 16 schematically illustrates a wafer that forms an array ofapertures for integration of lens carriers therein, in an embodiment.

FIG. 17 is an exploded view that schematically illustrates integrationof two lens wafers with a spacer wafer, in an embodiment.

FIG. 18 is an exploded view that schematically illustrates integrationof two lens wafers, in an embodiment.

FIG. 19 is a flowchart for a method of aligning and bonding twoelements, such as two lens wafers in an IR camera, with each other andwith a spacer wafer, in an embodiment.

FIG. 20 shows a cross section of an exemplary LWIR lens stack(configured with an object side lens element fabricated on a firstwafer, a stop and stray light control element, an image side lenselement fabricated on a second wafer, and a spacer, in an embodiment.

FIG. 21A illustrates exemplary performance of the lens stack of FIG. 20as a function of temperature.

FIG. 21B illustrates performance of the lens stack of FIG. 20 over afocus shift range as a function of temperature.

FIGS. 22, 24, 26, 28, 30, 32, 34, 36, 38, 40 and 42 illustrateathermalized lens configurations, in embodiments.

FIGS. 23A, 23B, 25A, 25B, 27A, 27B, 29A, 29B, 31A, 31B, 33A, 33B, 35A,35B, 37A, 37B, 39A, 39B, 41A and 41B illustrate performance of the lensconfigurations of FIGS. 22, 24, 26, 28, 30, 32, 34, 36, 38 and 40respectively.

FIG. 42 shows two lens carriers that may be integrated into a lenssystem, in an embodiment.

FIG. 43 shows an alternative arrangement that forms a lens systemincluding a spacer that can provide optical spacing and an aperture stopbetween lens carriers, in an embodiment.

FIG. 44 illustrates performance of lens configurations having variouscombinations of object side and image side thermal glass constants.

DETAILED DESCRIPTION OF THE DRAWINGS

Introduction

Certain materials that are typically used for LWIR imaging have largechanges in optical and mechanical properties with temperature.Therefore, creating an athermalized imaging design is a larger concernfor IR imaging than visible light imaging, due to the relative increasedsensitivity to temperature of common LWIR materials in comparison tocommon visible spectrum materials. Another challenge is that the LWIRimaging industry cannot rely on components, processes, or testingequipment that have been developed for the extremely high volumes of thevisible imaging and display businesses. Although some of the generaltechniques that have been developed for visible wafer level solutionscan be transferred, the technology is not readily adapted to the LWIRspectrum without significant changes to the process and the lensarchitecture to compensate for the unique challenges of imaging in theLWIR spectrum.

Lens Carrier Based Manufacturing and Passive Alignment

FIG. 3 illustrates a parallel longwave infrared (“LWIR”) manufacturingprocess 300. FIG. 4 is a schematic perspective view showing oneexemplary lens stack 400 produced by process 300 of FIG. 3.

In process 300, FIG. 3, arrays of lens elements are simultaneouslyfabricated on carriers or “wafers,” stacked with an optional,athermalizing spacer “wafer,” passively aligned, bonded (optionally withthe spacer), diced into complete IR optical modules and mounted onto IRsensors. (The carriers and spacer elements utilized in the manufacturingtechniques herein are sometimes called “wafers” due to the similarity ofsuch manufacturing to semiconductor processing that is performed onwafers cut from crystal boules.) “Sensors” herein are sensor chips suchas microbolometer arrays that include a plurality, usually a largenumber, of pixels that are sensitive to incoming radiation and can forman image therefrom. The resulting lens stacks 400 thus enable apassively athermalized IR imaging system that does not rely oncomplicated arrangements such as nested barrels and/or moving parts forathermalized performance.

In step 310 of process 300, an array of lenses is fabricated in each oftwo or more wafers. Exemplary lens materials and prescriptions, andfurther details about how the lenses may be fabricated, are providedbelow. In step 320, a spacer wafer is fabricated. Step 320 is optional,as one or more of the lens wafers may be formed of sufficient thicknessthat a spacer wafer is not needed to provide appropriate spacing and/orappropriate coefficient of thermal expansion for an athermalized design.In step 330, the lens wafers are aligned with each other and optionallyabout the spacer wafer, and bonded thereto to form a wafer level lensassembly. Further details on alignment strategies that may be employedwithin step 330 are also provided below. In step 340, the wafer levellens assembly is diced into single lens assemblies. Step 350 is anoptional lens test step to eliminate defective lens assemblies beforethey are mated to other camera components. In step 360, the lensassemblies are assembled to other components to form cameras. In step370, the cameras are calibrated and tested.

Certain steps of process 300 may be performed in a different order thanis shown in FIG. 3. For example, steps 310 and 320 may be performed inany order. Also, when step 350 is performed as part of process 300, itmay be performed before or after dicing step 340 (that is, the waferlevel lens assemblies formed in step 330 may be tested individually inwafer form before step 340, or as individual units after step 340).

It should also be clear that the steps of process 300 may be adapted toadd further lens and/or spacer layers to a lens stack, as compared tothe two lens stack produced by the minimum steps of process 300. Forexample, a common variation of process 300 will be to bond two lenswafers and two spacer wafers, to produce a lens stack that has spacingbetween two lenses and between an image side one of the lenses and asensor. Another common variation of process 300 will be to bond a spaceratop the lens stack to form an input aperture stop. Addition of otherfeatures by repeating steps of process 300 or by adding other featuresthat are compatible with the wafer level manufacturing scheme of process300 are contemplated and fall within the scope of the presentdisclosure.

By forming arrays of lenses, with precise spacing on or within amonolithic wafer, the need for individual, active alignment of each lensstack is eliminated. A single alignment step between one or more lensarray wafers and one or more spacer wafers accurately positions alllenses with respect to one another, thus improving off-axis imageperformance relative to conventional lens assembly by reducing decenterand tilt errors. This effectively eliminates the need for activealignment of each lens, a step that is currently a time-consuming, laborintensive adjustment required for each lens (e.g., step 110 of process100 discussed above). By simplifying and grouping (e.g., across a wafer)the number of assembly steps involved, lens carrier and/or lens waferbased fabrication also enables production of IR cameras that are morecompact and less costly than in the prior art. One key to cost reductionis to utilize more costly optical materials only where needed and toutilize appropriate, less costly base materials for structural supportof the optical materials. Methods of doing so, and the cameras formedthereby, are now discussed.

FIG. 4 schematically shows the major elements of a lens stack 400 and acamera 420 that can be produced utilizing process 300, FIG. 3. A portion402 of a first lens carrier includes a first lens 404. A portion 406 ofa spacer wafer is disposed between, and bonded to, overlying portion 402and an underlying portion 408 of a second lens carrier. Portion 408includes a second lens 410. Lens stack 400 includes portions 402, 406and 408, and can integrate with a sensor 412 to form camera 420.

FIGS. 5 through 8 schematically illustrate one way in which step 310 ofprocess 300, FIG. 3 may be performed, in an embodiment. FIG. 5illustrates a wafer 510 that forms an array of apertures 520 wherelenses will be fabricated. Each aperture 520 may be within an optionalrecess 530. Slugs 540 of a moldable material correspond to each aperture520 and are molded to form lenses therein, as explained below. Not allof apertures 520, recesses 530 and slugs 540 are labeled in FIG. 5, forclarity of illustration.

FIGS. 6 through 8 schematically illustrate how one slug 540 may bemolded into one of the apertures 520 of wafer 510 to form a lens, in anembodiment. FIG. 6 shows a portion of wafer 510 having a single aperture520 that is within recesses 530. A slug 540 is positioned withinaperture 520. Molds 600(1) and 600(2) that include mold surfaces 610(1)and 610(2) are shown above and below slug 540, respectively. Moldsurfaces 610(1) and 610(2) are complementary to desired surfaces of alens to be fabricated within aperture 520. FIG. 7 shows molds 600(1) and600(2) moved into position (e.g., in the directions shown by arrows,towards wafer 510) for molding about the aperture so as to mold the sluginto a lens 700. FIG. 8 shows lens 700 with molds 600(1) and 600(2)removed. Lens 700 forms surfaces 810(1) and 810(2) that arecomplementary to mold surfaces 610(1) and 610(2) respectively.

Numerous variations on the process illustrated in FIGS. 5 through 8 arecontemplated and/or would be considered equivalent to one skilled in theart. For example, in FIG. 8, lens 700 includes overmolded flanges 820(1)and 820(2) that may be advantageous for securing lens 700 within wafer510. Although two overmolded flanges are shown in FIG. 8, it iscontemplated that a molded lens could include only one, or no suchflanges. Alternatively, wafer 510 may form recesses, flanges orprotrusions that lens 700 can mold into or around, to secure the lenswithin the lens wafer.

In another example, instead of slugs 540, a moldable optical materialmay be injection molded to produce lens 700. Molds 600(1) and 600(2) maybe individual molds for one surface of one lens each, or may form partof molds that simultaneously provide mold surfaces 610(1) and 610(2) foreach of a plurality of lenses 700 that are concurrently fabricated inwafer 510. Surfaces 810(1) and 810(2) may be molded so as to providetheir final optical surfaces, or one or more of surfaces 810(1) and810(2) may be machined (e.g., diamond turned) to provide final opticalsurfaces.

Moreover, molding lenses into a wafer 510 is but one example of step310, FIG. 3, for providing a lens wafer. Another way to provide a lenswafer includes first providing lenses, either alone or within individuallens carriers, then assembling the lenses (and carriers, if used) intothe wafer. The assembly of the lenses and/or carriers into the wafer mayutilize molding and/or adhesives.

FIGS. 9 through 13 schematically illustrate a process for molding a lenscarrier or a lens wafer about one or more lenses, respectively. FIGS. 9and 10 show a lens 900 that is integrated with a circumferentialassembly flange 905. FIG. 10 is a bottom plan view looking upwardthrough lens 900, and FIG. 9 is a cross-sectional view taken at line9-9′ shown in FIG. 10. Lens 900 is for example injection molded toprovide lens surfaces 910(1) and 910(2) and assembly flange 905 thatincludes an alignment feature 915. Alignment feature 915 is shown inFIGS. 9 and 10 as a groove, but it is contemplated that alignmentfeature 915 could be one or more of (a) one or more indentations, suchas groove sections, or (b) protrusions, that can mate with complementaryfeatures of a lens carrier or lens wafer to align lens 900 to therespective carrier or wafer. Lens 900 and features thereof may be madeby molding or machining bulk material, or by a combination of moldingand machining before or after lens 900 is incorporated in a carrier orwafer. In particular, lens 900 may be molded to a shape wherein lenssurfaces 910(1) and 910(2) are outside the final desired surface, suchlens 900 may be incorporated into a carrier, then diamond turned toprovide the final desired surface. Alignment feature 915 can also beadded by machining before or after lens 900 is incorporated in a carrieror wafer, instead of being molded into lens 900.

FIGS. 11 and 12 show schematic cross-sections of a mold set includingupper and lower molds 1010(1) and 1010(2) that are configured to moldbase material to form a lens carrier about lens 900. FIG. 11 shows molds1010(1) and 1010(2) separated from lens 900, while FIG. 12 shows molds1010(1) and 1010(2) in contact with lens 900. FIG. 12 does not labelindividual features of molds 1010(1), 1010(2) and lens 900, for clarityof illustration. Upper mold 1010(1) includes a dam 1030(1) forcontacting assembly flange 905 of lens 900 to keep molded base materialaway from lens surface 910(1). Lower mold 1010(2) includes a similar dam1030(2) for contacting assembly flange 905 to keep molded base materialaway from lens surface 910(2), and a dam 1040 to form the outer edge ofthe lens carrier. A suitable base material would be injected into cavity1200, as shown in FIG. 12 with upper and lower molds 1010(1) and 1010(2)contacting lens 900, to form the lens carrier. Lower mold 1010(2) alsoincludes alignment features 1015(1) and 1015(2) that mate with alignmentfeature(s) 915 of lens 900, for precision alignment of final surfacesformed by lower mold 1010(2) relative to lens surfaces 910(1) and 910(2)of lens 900. Regions between alignment features 1015(1) and 1015(2) anddam 1030(2) are also considered portions of cavity 1200, as shown,because alignment features 1015(1) and 1015(2) may not extend fullyabout lens 900 (that is, alignment features 1015(1) and 1015(2) areshown for illustrative purposes in the cross-sectional plane shown inFIGS. 11 and 12, but may only exist at selected positions about acircumference of lens 900).

Lower mold 1010(2) also includes features 1035(1) and 1035(2) to providealignment features in the lens carrier. Because alignment features1015(1), 1015(2), 1035(1) and 1035(2) are all features of lower mold1010(2), alignment of lens 900 to lower mold 1010(2) is sufficient toprovide precision alignment of lens 900 (especially, lens surfaces910(1) and 910(2) thereof) to known features in the finished lenscarrier; that is, alignment of upper mold 1010(1) may not be ascritical. However, it is contemplated that upper mold 1010(1) could alsoinclude physical alignment features for precision alignment to lens 900(and lens 900 could include appropriate mating features therefor), orall of molds 1010(1), 1010(2) and lens 900 could include such features.

It should be clear upon reviewing and understanding FIGS. 11 and 12 thatmolds similar to upper and lower molds 1010(1) and 1010(2), but withoutdam 1040, and replicated to mold around an array of lenses, may beutilized to mold an entire lens wafer instead of a lens carrier.

FIGS. 13 and 14 show a schematic cross section of a lens carrier 1300produced by molding an appropriate base material 1310 around lens 900,using upper and lower molds 1010(1) and 1010(2) as shown in FIGS. 11 and12. FIG. 14 is a top plan view looking downward, and FIG. 13 is across-sectional view taken at line 13-13′ shown in FIG. 14. Basematerial 1310 is produced by molding a moldable base material withincavity 1200, FIG. 12, including the regions shown as alignment features1015(1) and 1015(2) in FIG. 11, because these features may only exist inselected positions about a circumference of lens 900. As shown in FIG.13, lens carrier 1300 includes alignment features 1315(1) and 1315(2)for subsequent precision alignment within a lens wafer. Base material1310 may be a material that absorbs at least IR radiation, so that whenlens carrier 1300 or a portion thereof becomes part of a cameraassembly, stray IR radiation that impinges on base material 1310 isabsorbed instead of continuing to propagate through the camera. Basematerial 1310 may also absorb visible and/or ultraviolet (“UV”)radiation. However, in some embodiments base material 1310 transmitsvisible and/or UV radiation, which can enhance manufacturability byfacilitating cure of certain epoxies.

FIG. 15 schematically illustrates a lens carrier 1300(2) produced bymounting lens 900 to a lens carrier blank 1510 utilizing an adhesive1517. Lens carrier blank 1510 may be made of a base material such asmetal (e.g., aluminum) that is not necessarily moldable under conditionsthat lens 900 would survive. Lens carrier blank 1510 may be made forexample by casting or by machining metal bar stock, and includesfeatures for aligning lens 900 and for later aligning completed lenscarrier 1300(2) to a lens wafer. Adhesive 1517 may be for example epoxythat is curable by applying visible or UV radiation; when such a curableepoxy is utilized, it may be advantageous to form lens carrier 1300(2)of a material that is transparent to the wavelength of radiationutilized to cure the epoxy. The amount and location of adhesive 1517 isexemplary; other embodiments of a lens carrier may utilize differentamounts or placement of such adhesive. Alternatively, lens 900 maysimply be press-fit into lens carrier blank 1510 to form completed lenscarrier 1300(2) without any adhesive.

FIG. 16 schematically illustrates a wafer 1610 that forms an array ofapertures 1620 for integration of lens carriers 1300 therein. Lenscarriers 1300 (e.g., either of lens carriers 1300(1), 1300(2) orequivalents thereof) correspond to each aperture 1620 and are mountedtherein utilizing any one or combination of mechanical fit, adhesives(such as epoxy), welding, soldering, and/or mating mechanical featuresassociated with the lens carriers and the apertures. Not all ofapertures 1620 and lens carriers 1300 are labeled in FIG. 16, forclarity of illustration.

FIG. 17 is an exploded view that schematically illustrates integrationof two lens wafers 1700, 1700′ with a spacer wafer 1780, as in step 330of process 300, FIG. 3. Lens wafers 1700, 1700′ are populated withlenses 1710, 1710′ respectively and may be fabricated as described inconnection with FIGS. 5 through 8 or FIGS. 9 through 16, in embodiments.Alternatively, either of lens wafers 1700, 1700′ may be entirely moldedand/or machined out of a single piece of a suitable optical material.Fabricating an entire lens wafer from optical material consumes more ofthe optical material itself, which may be costly, and can reduce theopportunity to utilize a base material that absorbs stray radiation, asnoted above. Lens wafer 1700 has at least a substantially planar uppersurface 1702 and lens wafer 1700′ has at least a substantially planarlower surface 1702′, except for respective alignment features 1715 and1715′, discussed below.

Spacer wafer 1780 has substantially planar, counterfacing surfaces 1782and 1784, separated by a thickness d. Apertures 1786 are shown in FIG.17 and are usually formed in spacer wafer 1780 for mechanical clearancefor lenses 1710, 1710′, and/or to allow IR (and/or visible radiation) topass without refraction or absorption between lenses 1710 and 1710′.Apertures 1786 may be omitted if spacer wafer 1780 is formed of an IR(and/or visible radiation) transparent material, if mechanical clearanceis not needed and if refraction by spacer wafer 1780 is included in theoptical prescription of imaging optics thus formed. Apertures 1786 areshown as rounded rectangles in FIG. 17, but may be of other shapes suchas circles that are matched to an optically active area of the imagingoptics. Spacer wafer 1780 may be formed for example of metal (e.g.,aluminum, steel, titanium, brass, copper or alloys thereof), plastic,ceramic or composite materials that may be selected on the basis ofmechanical strength, coefficient of thermal expansion, and/orabsorptivity with respect to IR or visible radiation (e.g., to absorbstray radiation, as discussed above).

Lens wafers 1700, 1700′ also include alignment features 1715, 1715′ thatare utilized for passive alignment to one another, as discussed below.Also shown in FIG. 17 are optional intermediate alignment elements 1720.Spacer wafer 1780 forms apertures 1788 that correspond with alignmentelements 1720, allowing alignment elements 1720 to contact bothalignment features 1715 and 1715′. Apertures 1788 are large enough toprovide clearance around alignment elements 1720 such that spacer wafer1780 remains free to move somewhat in the planes defined by the upperand lower surfaces of the spacer wafer. (Not all of lenses 1710, 1710′,apertures 1786, 1788 or alignment features 1715, 1715′ are labeled inFIG. 17, for clarity of illustration.) The clearance of apertures 1788relative to alignment elements 1720 avoids overconstraining alignmentelements 1720, so that alignment elements 1720 align solely withalignment features 1715, 1715′, yet constrains alignment of spacer wafer1780 so that the material of spacer wafer 1780 around each aperture 1786can be utilized as an aperture stop in a resulting lens assembly orcamera. For example, alignment features 1715, 1715′ may be conical orspherical indentations in lens wafers 1700, 1700′ respectively, andintermediate alignment elements 1720 may be spheres such as precisionball bearings. In another embodiment, alignment features 1715, 1715′ maybe oval or oblong features that provide additional degrees of freedomfor alignment elements 1720, as discussed below in connection with FIG.18. In still other embodiments, intermediate alignment elements 1720 arenot present, and alignment features 1715 and/or 1715′ protrudesubstantially from their respective surfaces through apertures 1788 andmate directly with each other. Alignment features 1715, 1715′ may beformed in lens wafers 1700, 1700′ by machining, molding, etching, orprinting, for example.

When lens wafers 1700, 1700′ are assembled with spherical alignmentelements 1720 and spacer wafer 1780, lens wafer 1700′ will naturallysettle into precise alignment with lens wafer 1700 without the use of anactive alignment step (e.g., using a mask aligner or other positioningdevice). When the opposing surfaces of lens wafers 1700, 1700′ areperfectly planar, intermediate alignment elements 1720 are perfectlyspherical, and three each of alignment features 1715, 1715′ withdiameters that match alignment elements 1720 are formed in anequilateral triangle in lens wafers 1700, 1700′, the lens wafers will beperfectly positioned and constrained in all six degrees of freedom.Adhesives or localized bonding techniques (e.g., localized welding orsoldering, performed on the stacked wafers as a unit or repeated foreach of the lens and spacer sets formed by the wafers) are utilized tobond lens wafers 1700, 1700′ with spacer wafer 1780 such that thealignment of lens wafers 1700, 1700′ remains precise across all of thelens and spacer sets thus formed.

It should also be noted that dimensions of alignment features 1715,1715′ and/or a size of intermediate alignment elements 1720 set aspacing between lens wafers 1700, 1700′ that is at least thickness d ofspacer wafer 1780, and which spacing may exceed thickness d. This allowsfor application of an adhesive (e.g., epoxy) between base lens wafer1700 and spacer wafer 1780, and between spacer wafer 1780 and upper lenswafer 1700′. In this manner, each set of elements (a lens in each oflens wafers 1700, 1700′ and a portion of spacer wafer 1780) are bondedand can be diced with the spacing intact. Other attachment means (e.g.,local welding, soldering) may also be utilized.

FIG. 18 is an exploded view that schematically illustrates integrationof two lens wafers 1700(2), 1700(2)′, also as in step 330 of process300, FIG. 3, but without showing a spacer wafer, for clarity ofillustration. Lens wafers 1700(2), 1700(2)′ are populated with lenses1710, 1710′ respectively and may be fabricated as described inconnection with FIGS. 5 through 8 or FIGS. 9 through 16, or may beentirely molded and/or machined out of a single piece of a suitableoptical material, in embodiments. Lens wafers 1700(2), 1700(2)′ aresimilar to lens wafers 1700, 1700′ except for alignment features 1715(2)and 1715(2)′ formed therein, respectively. Alignment features 1715(2)formed in lens wafer 1700(2) are shown as grooves that are radial withrespect to the center of lens wafer 1700(2), and are matched in width tointermediate alignment elements 1720. Alignment features 1715(2)′ formedin lens wafer 1700(2)′ are shown as semispherical depressions likealignment features 1715′. Since alignment features 1715(2) have a widththat matches intermediate alignment elements 1720 but with a length thatallows for some motion, lens wafer 1700(2) can be brought into andremain in rotational alignment with lens wafer 1700(2)′ with some roomfor expansion and/or contraction of either of the lens wafers. This canbe advantageous for testing individual pairs of lenses 1710, 1710′ inwafer form, which is often more economical than testing individualunits. For example, as is known in semiconductor fabrication, testing inwafer form can often be done on a sample basis to statistically verifythe performance of tested units without testing each one. Such testingcan also be done across lens wafers 1700(2), 1700(2)′ at varioustemperatures to verify an exact temperature to which the lens wafersshould be brought when they are joined to form a plurality of lensstacks that will later be singulated into individual lens stacks.

Although alignment features 1715(2) are shown as radial grooves thatallow expansion and alignment features 1715(2)′ are shown assemispherical depressions that fix positions of intermediate alignmentelements 1720, it is appreciated that these features may vary. Inparticular, an upper wafer may include grooves while a lower element maycontain depressions, or alignment features 1715(2) and 1715(2)′ may bedesigned to cooperate with each other directly rather than utilizingintermediate alignment elements 1720. For example, alignment elements1715(2) may be grooves as illustrated in FIG. 18, with alignmentelements 1715(2)′ being semispherical or conical protrusions.

FIG. 19 is a flowchart for a method 1900 of aligning and bonding twoelements, such as two lens wafers in an IR camera, with each other andwith a spacer wafer, as shown in FIG. 17. Method 1900 may be utilizedfor example as step 330 of process 300, FIG. 3. An optional step 1910can for example utilize lens wafer 1700 as a base element, and coupleintermediate alignment elements 1720 with alignment features 1715 oflens wafer 1700. An optional step 1920 applies one or more adhesives tothe base, middle, and/or upper elements. In an embodiment, an adhesiveis applied to the base and/or middle element so that they adhere whenthey come into contact. In another embodiment, the adhesive is curableand is applied to one or more of the base, middle, and/or upper elementsso that the elements may be brought into contact with one another butthe adhesive does not immediately bond them into place, allowing forsmall alignment adjustments (such as facilitated by mating alignmentelements 1715 of the base and upper elements). Step 1920 may be omittedwhen step 1950, below, adheres all of the base, middle and upperelements at once. Step 1930 places a middle element (e.g., spacer wafer1780) onto the base element. Step 1940 places a top element (e.g., lenswafer 1700′) over the middle element such that alignment features of thetop element engage alignment features of the base element or theintermediate alignment elements, if placed in step 1910. Step 1950 bondsthe top element to the base element and to the middle element, forexample by curing a curable epoxy applied in step 1920. It may beadvantageous to utilize lens wafers, spacer wafers and/or lens carriersthat are transparent to a curing wavelength of such a curable epoxy, forexample when IR lens elements are not transparent to the curingwavelength. Alternatively, in embodiments it is possible to utilizelocalized welding, soldering or mechanical attachments to bond the topelement to the base element, keeping in mind that the attachments mustremain secure while and after the lens stacks so formed are singulated.

In addition to lens wafer based manufacturing as discussed above, it isappreciated that lens carriers may be utilized in manufacturingindividual lens systems. For example, FIG. 42 shows two lens carriers1300(3), 1300(4) that may be integrated into a lens system 4200, similarto lens stack 400 (FIG. 4). Lens carriers 1300(3), 1300(4) may be madeby the techniques discussed above, especially in connection with FIG. 5through FIG. 12. Lens carriers 1300(3), 1300(4) do not include alignmentfeatures that mate with features of corresponding parts, but are formedwith a precise outside diameter so that they can be stacked within acylindrical alignment jig 4210 that holds them in alignment, as shown.Lens carriers 1300(3), 1300(4) may then be bonded together in alignmentwith one another to form lens system 4200. It is appreciated that thetechnique disclosed here can be utilized not only with circular carriersand a cylindrical alignment jig, but are easily adaptable to lenscarriers and corresponding alignment jigs that are square orrectangular, or are circular with keyed features that force theirpositioning into a known rotational alignment. Alignment jigs 4210 maybe removable or may form part of the finished lens systems. Alignmentjigs 4210 may facilitate bonding of lens carriers 1300(3), 1300(4) toone another (e.g., may be transparent to a curing wavelength of anepoxy, may transfer heat to lens carriers 1300(3), 1300(4) for weldingor soldering, or may form a mold for molding a further material aboutlens carriers 1300(3), 1300(4)).

FIG. 43 shows an alternative arrangement that forms a lens system 4300including a spacer 4220 that can provide optical spacing and an aperturestop between lens carriers 1300(3), 1300(4); spacer 4220 can also bemade of a material with a coefficient of thermal expansion that providesathermalization of lens system 4300. The lateral alignment of spacer4220 may not be as critical as the alignment of lens carriers 1300(3),1300(4) to one another, because optically it serves only to providespacing and possibly as an aperture stop for lens system 4300. Thereforespacer 4220 can be manufactured to a slightly smaller diameter andlooser diameter tolerance than lens carriers 1300(3), 1300(4). Theaddition of spacer 4220 is but one modification of lens carrier basedmanufacturing. Those skilled in the art will appreciate that thefeatures discussed in the context of lens wafer based manufacturingelsewhere in this disclosure may also be adapted or modified for use inlens carrier based manufacturing.

LWIR Material Selection and Athermalization Techniques

The goal of athermalizing a system is to design is to provide imagingperformance that remains good across a temperature range. This is adifficult characteristic to achieve in current LWIR systems, for tworeasons. First, as discussed above, many materials commonly used in LWIRsystems are sensitive to temperature. Second, the typical applicationsfor LWIR imaging require operation over a large temperature range.Without athermalization, the result would be reduced image quality(e.g., a blurred image due to inability to hold focus) at temperaturesaway from some nominal temperature. This would be unacceptable forautomotive, outdoor security and military applications where largetemperature swings are commonplace.

There are two common methods for athermalizing an LWIR imaging system.The first uses an active mechanism similar to auto focus, but instead ofbeing distance dependent, it is temperature dependent. The second is amulti-component, “nested” lens barrel that leverages a material'scoefficient of thermal expansion (CTE) to shift the image plane towardsthe lens as the power of the elements are increased, and away when thepower is decreased, due to temperature changes. However, wafer basedmanufacturing combined with selection of materials used therein basedboth on their imaging and thermal expansion properties can yield apassively athermalized IR imaging system that does not require eitheractive focus correction or nested barrels, as now discussed.

A useful definition of an athermalized lens is one for which apolychromatic, through-focus modulation transfer function (“MTF”) is atleast 0.1 across a specified temperature range, at ½ of an opticalcutoff frequency of the lens (1/((F/#)*λ)). Performance of athermalizedlenses described herein meet this criterion.

One class of materials that may be used in embodiments is that ofchalcogenide glasses, several of which are known to be moldable and tohave high transparency to infrared radiation. Chalcogenide glasses thatmay be utilized in embodiments may be obtained from several sources. Oneis Amorphous Materials, Inc. of Garland, Tex., selling such glassesunder the trademarked name “AMTIR” (an acronym for Amorphous MaterialTransmitting Infrared Radiation). These include AMTIR-1 (Ge₃₃As₁₂Se₅₅),AMTIR-2 (AsSe), AMTIR-3 (GeSbSe), AMTIR-4 (AsSe), AMTIR-5 (AsSe) andAMTIR-6 (As₂S₃). Another supplier is SCHOTT North America Inc., whichsells chalcogenide glasses under the trade names IG2 (Ge₃₃As₁₂Se₅₅),IG3(Ge₃₀As₁₃Se₃₂Te₂₅), IG4 (Ge₁₀As₄₀Se₅₀), IG5 (Ge₂₈Sb₁₂Se₆₀) and IG6(As₄₀Se₆₀). Another supplier is Umicore Electro-Optic Materials,(Belgium) which sells a chalcogenide glass called GASIR®1 InfraredTransmitting glass.

Other materials that are IR transmissive and may be utilized includeplastic, glass, crystalline materials (e.g., germanium, galliumarsenide, zinc selenide), and salts.

It is also important to utilize aspheric optics to control aberrations,field curvature and astigmatism. In embodiments herein, an object sidemeniscus lens forms at least one aspheric surface and an image sidemeniscus lens forms two aspheric surfaces. In a two elementconfiguration it is important to have aspheric degrees of freedom onthree of the four surfaces. The aspheric surface on the front element,near an aperture stop, is used to control spherical aberration. The twoaspheric surfaces on the second lens are used to control higher orderfield curvature and astigmatism. These three aspheric surfaces arerequired to enable the passively athermalized two meniscus configurationthat provides high performance. A fourth asphere can optionally be usedto slightly increase performance or manufacturability.

FIG. 20 shows a cross section of an exemplary LWIR lens stack 2000(e.g., lens stack 400 of FIG. 4) configured with an object side opticalelement 2002 fabricated on a first wafer, a spacer 2004 that may alsoserve as an aperture stop and stray light control element, an image sideoptical element 2006 fabricated on a second wafer, and a spacer 2008.Lens stack 2000 forms an image at an image plane 2010. Lens stack 2000includes embedded athermalization materials that form a combination ofthe lens element materials and spacer materials for passiveathermalization. Lens stack 2000 is also shown coated with an IRblocking and protective external covering 2012, for example to blockstray IR radiation and/or to improve structural integrity of the lensstack. FIG. 21A illustrates exemplary performance of lens stack 2000 asa function of temperature, where each line 2102, 2104, and 2106 shows amodulation transfer function (“Modulation”) of lens stack 2000 at adifferent temperature. FIG. 21B illustrates exemplary performance oflens stack 2000 at 50 cycles/mm (which is half the optical cutoff for anF/l lens) over a focus shift range of −0.1 to +0.1 mm from best focus,where each line 2108, 2110 and 2112 shows modulation at a differenttemperature. The performance of lens stack 2000 meets the athermalizedlens criterion noted above, since at the zero focus position, none ofthe different temperature curves drops below a modulation of 0.1.

In the example of FIGS. 20, 21A and 21B, an object side AMTIR-4 opticalelement 2002, a germanium image side optical element 2006, and Al 6061spacers 2004, 2008 are used in combination with the thermally dependentoptical power and thicknesses designed into optical elements 2002 and2006 to balance thermally induced focus errors through temperature.Optical elements 2002 and 2006 are both meniscus lenses with asphericsurfaces. In this configuration, optical element 2002 is considered anobject side meniscus lens (because it is closest to the object(s) beingimaged) and optical element 2006 is considered an image side meniscuslens. By embedding the athermalization in this way, lens stack 2000 doesnot require an athermalizing barrel or an expensive focus device, andcan be built as lens stacks formed from a lens wafer and spacer wafers,as discussed above.

Spacer elements can be utilized advantageously for stray light controlin embodiments. In prior art lens stacks (e.g., lens stack 200, FIG. 2),stray light is typically controlled by the combined characteristics ofthe barrel and the lenses. In low cost prior art imagers, where straylight is not specifically controlled and may therefore arrive at theimage plane, the imager may have reduced image contrast. In embodimentsherein, spacers not only form at least a part of the athermalizationarchitecture, but may also act as stray light baffles. For example, inFIG. 20, spacers 2004 and 2008 of lens stack 2000 allow light rays 2020(solid lines) to pass through optically active portions of opticalelements 2002 and 2006 and on to image plane 2010, but out-of-fieldlight rays 2030 (dashed lines) that enter the system are blocked by thespacers.

In the example of FIG. 20, lens stack 2000 has a volume (2.87 cm³) thatis 40% smaller than its closest commercially available counterpart(which is not entirely passively athermalized, and requires additionalcomponents beyond those used in this analysis). For this example, thefirst order parameters of the two solutions have been scaled to beequivalent for a fair comparison. The use of a barrel in the prior artcommercial solution adds significant size, whereas the use of protectiveexternal covering 2012 on the sides of lens stack 2000 substantiallyreduces the size of lens stack 2000, as compared to commerciallyavailable counterparts that use a lens barrel for the same purpose.External protective covering 2012 may be made from a variety ofmaterials, including IR blocking paints and coatings, or a low costmetal sleeve that is press fit over the entire lens stack 2000.

Another advantage provided by the manufacturing techniques herein isthat hermetically sealed units can be produced thereby. A typicaluncooled IR sensor is a microbolometer that requires a hermeticallysealed environment for long term reliability. Nested barrel type lenssystems typically cannot be hermetically sealed; prior art IR camerastypically utilize a window made of IR transparent material such asgermanium or gallium arsenide to seal a microbolometer into a cavitybefore mounting the sealed sensor to the nested barrel lens system. Thewindow adds cost and can generate a loss in signal due to Fresnelreflections from the high refractive index of the IR transparentmaterial.

Several advantageous strategies may be utilized to design an IR imagingsystem that is passively athermalized. For example, it may beadvantageous to constrain thermal glass constants of the object andimage side lenses. A thermal glass constant is defined as

${T_{gi} = {\frac{{\mathbb{d}n_{i}}/{\mathbb{d}T}}{( {n_{i} - 1} )} - \alpha_{i}}},$where

-   -   i is an index variable, that is, i=1 or 2 for the object and the        image side meniscus lenses respectively,    -   n_(i) is a refractive index of lens i,    -   α_(i) is a thermal expansion coefficient of lens i, and    -   T is temperature.

When considering an objective that consists of two elements, the changein focus due to temperature is approximated by:

${{dEFL} = {{- \frac{{EFL}*\Delta\; T}{4}}( {{3{CTE}} + {2T_{g\; 1}} + T_{g\; 2}} )}},$where

-   -   EFL=system focal length and dEFL=a change in system focal        length,    -   ΔT=a change in temperature    -   CTE=CTE of a spacer material between the image side and object        side meniscus lenses    -   T_(g1), T_(g2) are the thermal glass constants of the object and        the image side meniscus lenses, respectively.

Given these definitions, the approximate condition for an athermalizedobjective is:

$T_{g\; 1} = {{- \frac{1}{2}}{( {{3{CTE}} + T_{g\; 2}} ).}}$

In exemplary embodiments herein, T_(g1) is at least 26×10⁻⁶ less thanTg2, and more particularly, T_(g1) may be at least 60×10⁻⁶ less thanTg2. These conditions result in . . . (Need an explanation of why theseconditions are favorable . . . perhaps scatter chart of delta Tg vs.something useful, as we discussed?)

Lens stack 2000, FIG. 20, is an example of a system where T_(g2) is atleast 1.67 times T_(g1). Parameters for the object side meniscus lens(i.e., surfaces 1 and 2) and the image side meniscus lens (i.e.,surfaces 4 and 5) are given in Table 1. The material between the objectside and image side lenses in this and in all other examples below, isaluminum.

TABLE 1 Example 1 lens stack parameters, with object side meniscus lensformed of AMTIR-4 and image side lens formed of Germanium Surf RadiusThickness Glass Diameter 1 14.02745 2.002162 AMTIR-4 11.79292 2 19.78891.017775 10.48084 STO Infinity 8.611625 9.6 4 12.51413 2.998523GERMANIUM 14.28097 5 16.27892 4.112299 11.7186 T_(g2) − T_(g1) 171.4Surf Coeff on r4: Coeff on r6: Coeff on r8: 1 2.16E−05 −3.94E−073.74E−10 2 2.83E−05   2.63E−07 −5.20E−09 STO 4 0.000272422  −290E−063.58E−08 5 0.000778757  −126E−05 1.49E−07

Another example of a system where T_(g2) is at least 1.67 times T_(g1)is shown in FIG. 22. Only the object side meniscus lens 2210, image sidemeniscus lens 2220, and exemplary rays passing through the lens systemto an image plane 2230 are shown in FIG. 22. Performance of the systemshown in FIG. 22 is illustrated in FIG. 23A and FIG. 23B; each of thelines of each graph illustrates performance at one of temperatures −40C, 10 C and 60 C. The performance of the system meets the athermalizedlens criterion noted above meets the athermalized lens criterion notedabove, since at the zero focus position, none of the differenttemperature curves drops below a modulation of 0.1. Parameters forobject side meniscus lens 2210 and image side meniscus lens 2220 aregiven in Table 2.

TABLE 2 Example 2 lens stack parameters, with object side meniscus lensformed of AMTIR-2 and image side lens formed of Gallium Arsenide SurfRadius Thickness Glass Diameter 1 13.24067 2.001792 AMTIR-2 13.68846 218.44871 2.115521 12.56471 STO Infinity 6.967737 9.6 4 10.80241 3.0006GAAS 12.95704 5 14.5185 3.616475 10.57044 T_(g2) − T_(g1) 77.7 SurfCoeff on r4: Coeff on r6: Coeff on r8: 1 5.07E−06 1.07E−06 −4.66E−08 24.27E−05 8.23E−07 −6.79E−08 STO 4 0.000295523 −249E−06  3.57E−08 50.000978647 −1.14E−05   9.16E−08

Another example of a system where T_(g2) is at least 1.67 times T_(g1)is shown in FIG. 24. Only the object side meniscus lens 2410, image sidemeniscus lens 2420, and exemplary rays passing through the lens systemto an image plane 2430 are shown in FIG. 24. Performance of the systemshown in FIG. 24 is illustrated in FIG. 25A and FIG. 25B; each of thelines of each graph illustrates performance at one of temperatures −40C, 10 C and 60 C. The performance of the system meets the athermalizedlens criterion noted above meets the athermalized lens criterion notedabove, since at the zero focus position, none of the differenttemperature curves drops below a modulation of 0.1. Parameters forobject side meniscus lens 2410 and image side meniscus lens 2420 aregiven in Table 3.

TABLE 3 Example 3 lens stack parameters, with object side meniscus lensformed of AMTIR-2 and image side lens formed of Germanium Surf RadiusThickness Glass Diameter 1 14.201 1.994121 AMTIR-2 11.72873 2 19.403310.9784842 10.40591 STO Infinity 8.664542 9.6 4 12.51486 2.996764GERMANIUM 14.34517 5 16.29145 4.079677 11.78007 T_(g2) − T_(g1) 145.6Surf Coeff on r4: Coeff on r6: Coeff on r8: 1 2.17E−05 −2.89E−07−6.34E−10 2 2.76E−05   1.56E−07 1.33E−09 STO 4 0.000272429 −2.90E−063.57E−08 5 0.000777671 −1.26E−05 1.47E−07

Another example of a system where T_(g2) is at least 1.67 times T_(g1)is shown in FIG. 26. Only the object side meniscus lens 2610, image sidemeniscus lens 2620, and exemplary rays passing through the lens systemto an image plane 2630 are shown in FIG. 26. Performance of the systemshown in FIG. 26 is illustrated in FIG. 27A and FIG. 27B; each of thelines of the graph illustrates performance at one of temperatures −40 C,10 C and 60 C. The performance of the system meets the athermalized lenscriterion noted above. Parameters for object side meniscus lens 2610 andimage side meniscus lens 2620 are given in Table 4.

TABLE 4 Example 4 lens stack parameters, with object side meniscus lensformed of AMTIR-2 and image side lens formed of Silicon Surf RadiusThickness Glass Diameter 1 13.88453 2.001792 AMTIR-2 11.78243 2 19.069580.9562742 10.56639 STO Infinity 8.037013 9.6 4 12.12694 3.000315 SILICON13.46339 5 17.44629 4.186328 11.20679 T_(g2) − T_(g1) 82.3 Surf Coeff onr4: Coeff on r6: Coeff on r8: 1 −1.59E−06 −5.13E−07 −2.78E−08 2 2.42E−05−1.29E−06 −3.37E−08 STO 4 0.000275359 −2.81E−06 3.38E−08 5 0.000800679−1.31E−05 1.57E−07

Another example of a system where T_(g2) is at least 1.67 times T_(g1)is shown in FIG. 28. Only the object side meniscus lens 2810, image sidemeniscus lens 2820, and exemplary rays passing through the lens systemto an image plane 2830 are shown in FIG. 28. Performance of the systemshown in FIG. 28 is illustrated in FIG. 29A and FIG. 29B; each of thelines of each graph illustrates performance at one of temperatures −40C, 10 C and 60 C. The performance of the system meets the athermalizedlens criterion noted above. Parameters for object side meniscus lens2810 and image side meniscus lens 2820 are given in Table 5.

TABLE 5 Example 5 lens stack parameters, with object side meniscus lensformed of Zinc Selenide and image side lens formed of Germanium SurfRadius Thickness Glass Diameter 1 12.7184 2.000568 ZNSE 12.48927 218.97205 1.497532 11.22439 STO Infinity 7.944457 9.6 4 12.0265 3.000467GERMANIUM 13.70417 5 14.90827 4.013358 11.11599 T_(g2) − T_(g1) 90.5Surf Coeff on r4: Coeff on r6: Coeff on r8: 1 2.55E−05 9.27E−07−3.27E−08 2 7.92E−05 8.24E−07 −4.87E−08 STO 4 0.000281568 −2.77E−063.43E−08 5 0.000810695 −1.14E−05 1.30E−07

Another example of a system where T_(g2) is at least 1.67 times T_(g1)is shown in FIG. 30. Only the object side meniscus lens 3010, image sidemeniscus lens 3020, and exemplary rays passing through the lens systemto an image plane 3030 are shown in FIG. 30. Performance of the systemshown in FIG. 30 is illustrated in FIGS. 31A and 31B; each of the linesof each graph illustrates performance at one of temperatures −30 C, 10 Cand 50 C. The performance of the system meets the athermalized lenscriterion noted above, although the temperature range of this particularlens system is reduced as compared to other lenses disclosed herein.Parameters for object side meniscus lens 3010 and image side meniscuslens 3020 are given in Table 6.

TABLE 6 Example 6 lens stack parameters, with object side meniscus lensformed of Gallium Arsenide and image side lens formed of Germanium SurfRadius Thickness Glass Diameter 1 14.66519 2.0004 GAAS 12.46912 218.43001 1.432689 11.16288 STO Infinity 8.030219 9.6 4 12.52965 2.999147GERMANIUM 13.83558 5 16.19574 3.996068 11.34395 T_(g2) − T_(g1) 67.9Surf Coeff on r4: Coeff on r6: Coeff on r8: 1 1.07E−05 −2.05E−07−1.41E−08 2 3.41E−05 −6.22E−07 −1.84E−08 STO 4 0.000273252 −2.91E−063.50E−08 5 0.000768881 −1.27E−05 1.54E−07

Another example of a system where T_(g2) is at least 1.67 times T_(g1)is shown in FIG. 32. Only the object side meniscus lens 3210, image sidemeniscus lens 3220, and exemplary rays passing through the lens systemto an image plane 3230 are shown in FIG. 32. Performance of the systemshown in FIG. 32 is illustrated in FIG. 33A and FIG. 33B; each of thelines of the graph illustrates performance at one of temperatures −40 C,10 C and 60 C. The performance of the system meets the athermalized lenscriterion noted above. Parameters for object side meniscus lens 3210 andimage side meniscus lens 3220 are given in Table 7.

TABLE 7 Example 7 lens stack parameters, with object side meniscus lensformed of AMTIR-4 and image side lens formed of Gallium Arsenide SurfRadius Thickness Glass Diameter 1 13.7553 2.000217 AMTIR-4 11.30208 219.48751 0.6224783 10.177.08 STO Infinity 8.107027 9.6 4 12.18742.999143 GAAS 13.35351 5 18.36456 4.334889 11.19408 T_(g2) − T_(g1)103.5 Surf Coeff on r4: Coeff on r6: Coeff on r8: 1 −3.95E−06 −8.23E−07−3.22E−08 2 1.79E−05 −1.41E−06 −4.64E−08 STO 4 0.000273914 −2.88E−063.39E−08 5 0.000784306 −1.28E−05 1.55E−07

Another example of a system where T_(g2) is at least 1.67 times T_(g1)is shown in FIG. 34. Only the object side meniscus lens 3410, image sidemeniscus lens 3420, and exemplary rays passing through the lens systemto an image plane 3430 are shown in FIG. 34. Performance of the systemshown in FIG. 34 is illustrated in FIG. 35A and FIG. 35B; each of thelines of each graph illustrates performance at one of temperatures −40C, 10 C and 60 C. The performance of the system meets the athermalizedlens criterion noted above. Parameters for object side meniscus lens3410 and image side meniscus lens 3420 are given in Table 8.

TABLE 8 Example 8 lens stack parameters, with object side meniscus lensformed of AMTIR-4 and image side lens formed of AMTIR-2 Surf RadiusThickness Glass Diameter 1 13.41408 2.002164 AMTIR-4 11.89383 2 19.07760.9713252 10.82319 STO Infinity 7.267797 9.6 4 11.69409 2.99037 AMTIR-212.81551 5 21.42909 4.344554 11.16564 T_(g2) − T_(g1) 25.8 Surf Coeff onr4: Coeff on r6: Coeff on r8: 1 −3.74E−05 −1.06E−06 −4.55E−08 2−1.78E−05 −2.78E−06 −3.60E−08 STO 4 0.000279049 −2.82E−06 −6.75E−11 50.000881254 −1.61E−05 9.87E−08

Another example of a system where T_(g2) is at least 1.67 times T_(g1)is shown in FIG. 36. Only the object side meniscus lens 3610, image sidemeniscus lens 3620, and exemplary rays passing through the lens systemto an image plane 3630 are shown in FIG. 36. Performance of the systemshown in FIG. 36 is illustrated in FIG. 37A and FIG. 37B; each of thelines of each graph illustrates performance at one of temperatures −40C, 10 C and 60 C. The performance of the system meets the athermalizedlens criterion noted above. Parameters for object side meniscus lens3610 and image side meniscus lens 3620 are given in Table 9.

TABLE 9 Example 9 lens stack parameters, with object side meniscus lensformed of AMTIR-4 and image side lens formed of Silicon Surf RadiusThickness Glass Diameter 1 13.79301 2.00216 AMTIR-4 11.28861 2 19.532930.6495318 10.10299 STO Infinity 8.186761 9.6 4 12.25933 2.999695 SILICON16.99682 5 17.67549 4.313474 14.34624 T_(g2) − T_(g1) 108.1 Surf Coeffon r4: Coeff on r6: Coeff on r8: 1 3.82E−07 −8.27E−07 −3.94E−08 22.13E−05 −1.33E−06 −5.36E−08 STO 4 0.00027401 −2.88E−06 3.40E−08 50.000779558 −1.28E−05 1.60E−07

In another strategy to design an IR imaging system that is passivelyathermalized, it is advantageous to constrain two aspheric meniscuslenses such that an optical power of the image side meniscus lens is atleast 1.6 times an optical power of the object side meniscus lens. Thisis useful for both aberration correction and for athermalization. Also,when thermal glass constants of the two lenses are not constrained,certain resulting lens stacks may utilize two lenses of the samematerial, which may help in reducing cost.

An example of a system where an optical power of the image side meniscuslens is at least 1.6 times an optical power of the object side meniscuslens is shown in FIG. 38. Only the object side meniscus lens 3810, imageside meniscus lens 3820, and exemplary rays passing through the lenssystem to an image plane 3830 are shown in FIG. 38. Performance of thesystem shown in FIG. 38 is illustrated in FIG. 39A and FIG. 39B; each ofthe lines of each graph illustrates performance at one of temperatures−40 C, 10 C and 60 C. The performance of the system meets theathermalized lens criterion noted above. Parameters for object sidemeniscus lens 3810 and image side meniscus lens 3820 are given in Table10.

TABLE 10 Example 10 lens stack parameters, with object side meniscuslens and image side lens both formed of Silicon Surf Radius ThicknessGlass Diameter 1 14.60529 1.973347 SILICON 12.77556 2 18.07738 1.59493311.59552 STO Infinity 7.414748 9.6 4 12.12362 3.000314 SILICON 13.210685 17.39618 4.028685 11.17493 T_(g2) − T_(g1) 0 Surf Coeff on r4: Coeffon r6: Coeff on r8: 1 5.97E−06 −7.02E−07 −2.20E−08 2 3.09E−05 −1.91E−06−2.04E−08 STO 4 0.00026331 −2.94E−06 1.75E−08 5 0.00079123 −1.46E−051.07E−07

A further example of a system where T_(g2) is at least 1.67 times T_(g1)is shown in FIG. 40. (In this example, both of the lenses are formed ofAMTIR-2, which has a negative T_(g), so T_(g2) is not as negative as1.67 times T_(g1).) Only the object side meniscus lens 4010, image sidemeniscus lens 4020, and exemplary rays passing through the lens systemto an image plane 4030 are shown in FIG. 40. Performance of the systemshown in FIG. 40 is illustrated in FIG. 41A and FIG. 41B; each of thelines of each graph illustrates performance at one of temperatures −40C, 10 C and 60 C. The performance of the system meets the athermalizedlens criterion noted above. Parameters for object side meniscus lens4010 and image side meniscus lens 4020 are given in Table 11.

TABLE 11 Example 11 lens stack parameters, with object side meniscuslens and image side lens both formed of AMTIR-2 Surf Radius ThicknessGlass Diameter 1 10.78755 1.362582 AMTIR-2 12.18862 2 13.89166 1.81618311.22592 STO Infinity 7.200517 9.6 4 9.866962 3.00269 AMTIR-2 12.34325 514.30628 3.749132 10.04798 T_(g2) − T_(g1) 0 Surf Coeff on r4: Coeff onr6: Coeff on r8: 1 3.99E−05 6.17E−06 −4.67E−08 2 8.95E−05 1.03E−05−1.29E−07 STO 4 0.000362993 −5.56E−06 1.09E−07 5 0.001165077 −1.73E−053.17E−07

In exemplary embodiments herein, T_(g2) is greater than (1.67*T_(g1)) inthe solutions that meet the athermalization criteria noted above. Moreparticularly, T_(g2) may be greater than (1.25*T_(g1)−50×10⁻⁶), as canbe shown in FIG. 44. FIG. 44 has units of 10⁻⁶ and shows the topperforming material combinations, indicated by diamonds 4100, and theworst performing material combinations, indicated by squares 4102. Forthe sake of clarity, not all material combinations are numbered.

Combinations of Features

Features described above as well as those claimed below may be combinedin various ways without departing from the scope hereof. The followingexamples illustrate possible, non-limiting combinations the presentinvention has been described above, it should be clear that many changesand modifications may be made to the process and product withoutdeparting from the spirit and scope of this invention:

(a) A passively athermalized infrared imaging system includes an objectside meniscus lens that forms at least one aspheric surface, and animage side meniscus lens that forms two aspheric surfaces. Each of themeniscus lenses is formed of a material selected from the groupconsisting of a chalcogenide glass, germanium, silicon, galliumarsenide, zinc selenide and glass. An optical power of the image sidemeniscus lens is at least 1.6 times an optical power of the object sidemeniscus lens, such that an effective focus position of the imagingsystem is athermalized over a range of 0 to +40 degrees Celsius.

(b) In system denoted as (a), athermalized may be defined as apolychromatic, through-focus modulation transfer function (“MTF”) beingat least 0.1 over the range of 0 to +40 degrees Celsius, at ½ of anoptical cutoff frequency of the lens (1/((F/#)*λ)).

(c) In the system/s denoted as (a) or (b), a thermal glass constant ofthe object side meniscus lens may be equal to a thermal glass constantof the image side meniscus lens.

(d) In the system denoted as (c), the thermal glass constants may bedefined as

${T_{g\; i} = {\frac{{\mathbb{d}n_{i}}/{\mathbb{d}T}}{( {n_{i} - 1} )} - \alpha_{i}}},$wherei is 1 or 2 for the object and the image side meniscus lensesrespectively,n_(i) is a refractive index of lens i,α_(i) is a thermal expansion coefficient of lens i, andT is temperature.

(e) In the system/s denoted as (a)-(d), each of the object side meniscuslens and the image side meniscus lens may include chalcogenide glass.

(f) In the system/s denoted as (a)-(d), each of the object side meniscuslens and the image side meniscus lens may include silicon.

(g) In the system/s denoted as (a)-(f), the system may be free ofpowered optics except for the object side and image side meniscuslenses.

(h) In the system/s denoted as (a)-(d) and (h), the object side meniscuslens may be molded of a chalcogenide glass.

(i) In the system/s denoted as (a)-(h), a thermal image may be formed atan image plane. A distance between the image side meniscus lens and theimage plane is set by a first fixed spacer element therebetween, and adistance between the object side and image side meniscus lenses is setby a second fixed spacer element therebetween.

(j) In the system/s denoted as (a)-(i), the effective focus position ofthe imaging system may be athermalized over a range of −40 to +60degrees Celsius.

(k) A passively athermalized infrared imaging system includes an objectside meniscus lens, formed of a first material that transmits infraredradiation, that forms at least one aspheric surface; and an image sidemeniscus lens, formed of a second material that transmits infraredradiation, that forms two aspheric surfaces. The object side and imageside meniscus lenses cooperate to form a thermal image and being concavetowards the image. The first and second materials have thermal glassconstants T_(g1) and T_(g2) respectively, wherein T_(g2) is at least1.67 times T_(g1), such that an effective focus position of the systemis athermalized over a temperature range of 0 to +40 degrees Celsius.

(l) In the system denoted as (k), athermalized may be defined as apolychromatic, through-focus modulation transfer function (“MTF”) beingat least 0.1 over the range of 0 to +40 degrees Celsius, at ½ of anoptical cutoff frequency of the lens (1/((F/#)*λ)).

(m) In the system/s denoted as (k) or (l), the system may be free ofpowered optics except for the first and second lens.

(n) In system/s denoted as (k)-(m), the thermal image forms at an imageplane, and a distance between the second lens and the image plane may beset by a first fixed spacer element therebetween. A distance between thefirst lens and the second lens may be set by a second fixed spacerelement therebetween.

(o) In system/s denoted as (k)-(n), the first and second materials maybe selected from the group consisting of a chalcogenide glass,germanium, silicon, gallium arsenide, zinc selenide and glass.

(p) In system/s denoted as (k)-(o), each of the first and secondmaterials may be chalcogenide glass.

(q) In system/s denoted as (o), T_(g2) may be at least (1.25 timesT_(g1) plus 50×10⁻⁶), and each of the first and second materials may bea different one of said group.

(r) In system/s denoted as (k)-(o) or (q), the first material may be achalcogenide glass, and the second material may be germanium.

(s) In system/s denoted as (k)-(o) or (q), the first material may be achalcogenide glass and the second material may be gallium arsenide.

(t) In system/s denoted as (k)-(o) or (q), the first material may be achalcogenide glass and the second material may be silicon.

(u) In system/s denoted as (k)-(o) or (q), the first material may bezinc selenide and the second material may be germanium.

(v) In system/s denoted as (k)-(o) or (q), the first material may begallium arsenide and the second material may be germanium.

(w) In system/s denoted as (k)-(v), the effective focus position of theimaging system may be athermalized over a range of −40 to +60 degreesCelsius.

(x) A method of aligning elements in a manufacturing process includesplacing a middle element onto a base element, the base element formingfirst alignment features, the middle element forming aperturestherethrough corresponding to the first alignment features. Secondalignment features of an upper element are placed onto the firstalignment features such that the first and second alignment featurescooperate, through the apertures, to align the upper element with thebase element.

(y) In the method denoted as (x), the steps of placing the middleelement and the second alignment features aligns the middle element tothe base element and the upper element with a tolerance corresponding toa clearance of the apertures around the first and second alignmentfeatures.

(z) In the method/s denoted as (x) or (y), the base, middle and upperelements may be bonded together while the alignment features cooperate,to form a composite element; and the composite element may be diced togenerate a plurality of subunits that do not include any portion of thefirst or second alignment features, but in which respective portions ofthe base and upper elements remain in alignment.

(a1) In the method/s denoted as (z) bonding includes utilizing anadhesive.

(b1) A method of aligning elements in a manufacturing process includesplacing a middle element onto a base element. The base element formsfirst alignment features, and the middle element forms aperturestherethrough corresponding to the first alignment features. Intermediatealignment elements are placed upon the first alignment features; andsecond alignment features of an upper element are placed onto theintermediate alignment features such that the first, intermediate, andsecond alignment features cooperate to align the upper element with thebase element.

(c1) In the method denoted as (b1), placing the intermediate alignmentelements includes placing substantially spherical elements into recessesin the base element that form the first alignment elements.

(d1) In the method/s denoted as (b1) or (c1), the steps of placing themiddle element, the intermediate alignment elements and the secondalignment features may align the middle element to the base element andthe upper element with a tolerance corresponding to a clearance of theapertures around the intermediate alignment elements.

(e1) A plurality of infrared lens systems includes at least one firstlens wafer having a first base material that is opaque to infraredradiation, with infrared transmissive material inset into aperturestherein to form first lenses. The first lens wafer is bonded with atleast one second lens wafer having a plurality of second lenses, suchthat pluralities of the first and second lenses align to form the lenssystems.

(f1) In the plurality of systems denoted as (e1), the infraredtransmissive material may be inset into carriers that are inset into thefirst base material.

(g1) In the plurality of systems denoted as (e1) or (f1), the first basematerial may transmit at least one of visible and ultraviolet radiation.

(h1) In the plurality of systems denoted as (e1)-(g1), the second lenswafer may include a second base material that is opaque to infraredradiation, with infrared transmissive material inset into aperturestherein to form the second lenses.

(i1) In the plurality of systems denoted as (h1), the first basematerial may transmit at least one of visible and ultraviolet radiation.

(j1) In the plurality of systems denoted as (e1)-(i1), the first basematerial may be one of metal, plastic, ceramic and composite.

(k1) In the plurality of systems denoted as (e1)-(j1), the first andsecond wafers may be joined to one another.

(l1) In the plurality of systems denoted as (e1)-(k1), the first andsecond lens wafers may be mechanically joined.

(m1) In the plurality of systems denoted as (e1)-(l1), the first andsecond lens wafers may be joined with an adhesive.

(n1) In the plurality of systems denoted as (e1)-(m1), the first andsecond lens wafers may be diced to separate each of the lens systemsfrom one another.

(o1) In the plurality of systems denoted as (n1), after dicing, the lenssystems may not include alignment features utilized to align the firstand second lens wafers.

(p1) The plurality of systems denoted as (e1)-(o1) may further include asensor aligned to at least one of the lens systems.

(q1) In the plurality of systems denoted as (p1), the sensor may behermetically sealed to the at least one of the lens systems.

(r1) The plurality of systems denoted as (e1)-(o1) may further include aplurality of spacers, each of the spacers being disposed between andbonded with one of the first lenses and one of the second lenses, eachof the spacers forming an aperture along an optical axis formed by theones of the first and second lenses.

(s1) The plurality of systems denoted as (e1)-(q1) may further include aspacer layer forming a plurality of apertures between each of the firstand second lenses, the spacer layer being disposed between and bondedwith the first and second lens wafers.

(t1) In the plurality of systems denoted as (s1), the spacer layer maybe opaque to infrared energy.

(u1) In the plurality of systems denoted as (s1) or (t1), the spacerlayer may be one of metal, plastic, ceramic and composite.

(v1) The plurality of systems denoted as (e1)-(u1) may further include asensor array layer having a plurality of sensors, each sensor aligningto a corresponding one of the lens systems.

(w1) In the plurality of systems denoted as (v1), each of the sensorsmay be hermetically sealed with the corresponding one of the lenssystems.

(x1) In the plurality of systems denoted as (v1) or (w1), each of thesensors is hermetically sealed with the corresponding one of the lenssystems without a cover plate over the sensor array.

(y1) A lens wafer for use in optical manufacturing includes a substrateforming apertures therein, the substrate being formed of a basematerial; and a plurality of lenses, each of the lenses including anoptical material and disposed within a respective one of the apertures.

(z1) In the wafer denoted as (y1), the base material may be or includeone of metal, plastic, ceramic and composite.

(a2) In the wafer denoted as (y1) or (z1), each of the plurality oflenses may be at least 200 microns from each other of the plurality oflenses such that the substrate can be diced to singulate portions of thesubstrate having a single lens therein without damaging any of theplurality of lenses.

(b2) In the wafer denoted as (y1)-(a2), the optical material may betransparent to IR radiation.

(c2) In the wafer denoted as (y1)-(b2), the optical material may be orinclude one or more of a chalcogenide glass, a crystalline material, asalt, plastic or glass.

(d2) The wafer denoted as (y1)-(c2) may further include mechanicalalignment features therein for alignment of the lens wafer to a secondlens wafer.

(e2) In the wafer denoted as (y1)-(d2), each of the lenses may bedisposed within its respective aperture in a nonfinal form, and may besubsequently shaped to a final form while in the aperture.

(f2) In the wafer denoted as (e2), each of the lenses may be shaped tothe final form by one of polishing, grinding, diamond turning,magnetorheological finishing, compression molding or injection molding.

(g2) In the wafer denoted as (y1)-(f2), each of the plurality of lensesmay form an outwardly facing flange, and the substrate may be or includeplastic that is overmolded about each of the flanges.

(h2) In the wafer denoted as (g2), each of the outwardly facing flangesmay form an alignment feature that cooperates with a mold, to align eachrespective lens within the lens wafer as the plastic is overmolded abouteach of the flanges using the mold.

(i2) In the wafer denoted as (g2), the substrate may include alignmentfeatures for subsequent alignment of the substrate, and the alignmentfeatures may be formed as the plastic is overmolded about each of theflanges using the mold.

(j2) In the wafer/s denoted as (y1)-(g2), each of the plurality oflenses may be held by a respective lens carrier.

(k2) In the wafer denoted as (j2), the lens carriers may be held withinrespective apertures of the substrate with an adhesive.

(l2) In the wafer/s denoted as (j2) or (k2), each of the lens carriersmay be or include one or more of metal, plastic and ceramic.

(m2) In the wafer/s denoted as (j2)-(12), each of the lens carriers mayinclude alignment features for alignment of the lens carriers with thelens wafer.

(n2) An infrared lens assembly includes a lens formed of an infraredtransmitting material that is disposed within a carrier of a basematerial, the lens being molded within the carrier with at least onefeature that secures the lens to the carrier.

(o2) In the assembly denoted as (n2), the lens may include one or moreof a chalcogenide glass, plastic, glass, a crystalline material, and asalt.

(p2) In the assembly/ies denoted as (n2) and (o2), the carrier may formone of a recess and a protrusion that engages the at least one featureof the lens.

(q2) In the assembly/ies denoted as (n2)-(p2), the carrier may form atleast one alignment feature for aligning the carrier to another carrier.

(r2) In the assembly/ies denoted as (n2)-(q2), the base material mayinclude plastic.

(s2) In the assembly/ies denoted as (n2)-(q2), the base material maytransmit visible radiation and be opaque to infrared radiation.

(t2) A mold set includes an upper mold and a lower mold, at least one ofthe upper and lower molds having one or more features that areconfigured to hold an infrared lens in an aligned position, the upperand lower molds configured to provide a cavity for molding a moldablematerial into one of a lens carrier and a lens wafer about the infraredlens.

(u2) In the mold set denoted as (t2), at least one of the upper andlower molds may include a dam for preventing the moldable material fromobscuring an optical surface of the infrared lens.

(v2) An infrared lens assembly includes a lens formed of an infraredtransmitting material that is disposed within a carrier of a basematerial, the carrier being molded around the lens with at least onefeature that secures the lens to the carrier.

(w2) An infrared lens stack includes a first lens formed of an infraredtransmitting material and disposed within a first carrier, formed of abase material, that forms at least one first alignment feature; and asecond lens formed of an infrared transmitting material and disposedwithin a second carrier, formed of a base material, that forms at leastone second alignment feature configured to cooperate with the firstalignment feature.

(x2) In the infrared lens stack/s denoted as (v2) or (w2), the carriersmay be bonded together.

(y2) In the infrared lens stack/s denoted as (v2)-(x2), any of the basematerials may include plastic.

(z2) In the infrared lens stack/s denoted as (v2)-(y2), plastic maytransmit at least one of visible and ultraviolet radiation but be opaqueto infrared radiation.

Changes may be made in the above methods and systems without departingfrom the scope hereof. It should thus be noted that the matter containedin the above description or shown in the accompanying drawings should beinterpreted as illustrative and not in a limiting sense. The followingclaims are intended to cover all generic and specific features describedherein, as well as all statements of the scope of the present method andsystem, which, as a matter of language, might be said to falltherebetween.

What is claimed is:
 1. A method of aligning upper and lower wafers, thelower wafer having a plurality of first optical elements eachrespectively opposing one of a plurality of second optical elements ofthe upper wafer, and the lower wafer having a plurality of firstalignment features each respectively opposing one of a plurality ofsecond alignment features of the upper wafer, the method comprising:positioning a spacer wafer, the spacer wafer having a plurality ofalignment apertures each respectively corresponding one of the pluralityof first alignment features, on the lower wafer such that the pluralityof alignment apertures align with the plurality of first alignmentfeatures; inserting, after the step of positioning, one of a pluralityof alignment elements within the each of the alignment apertures;arranging the upper wafer on the spacer wafer such that the upper andlower wafers passively align via the plurality of alignment elements;wherein the first alignment features, and the respective first andsecond alignment features cooperate to passively align the upper waferto the lower wafer in at least four degrees of freedom, the four degreesof freedom including: X- and Y-coordinates defining centering of eachone of the plurality of first optical elements to a respective one ofthe plurality of second optical elements, a Z-coordinate definingspacing between the lower wafer and the upper wafer, and rotation of thelower wafer with respect to the upper wafer.
 2. The method of claim 1,wherein the plurality of first and/or second alignment features areprotrusions from each respective lower and/or upper wafer and serve asthe plurality of alignment elements.
 3. The method of claim 1, theplurality of first and second alignment features including conicalindentations, spherical indentations, oval features, oblong features,grooves, radial grooves, and/or semispherical depressions of the lowerand upper wafers.
 4. The method of claim 1, the step of arrangingincluding positioning the upper wafer such that the plurality of firstand second alignment features respectively make contact to a respectiveone of the plurality of alignment elements.
 5. The method of claim 1,the alignment elements having a first width that is smaller than asecond width of the alignment apertures of the spacer wafer.
 6. Themethod of claim 1, the plurality of first alignment features, theplurality of second alignment features and the plurality of alignmentapertures all being spaced apart near respective edges of the lowerwafer, upper wafer, and spacer wafer, respectively.
 7. The method ofclaim 6, wherein the plurality of first alignment features, theplurality second alignment features and the plurality of alignmentapertures are spaced apart to substantially form points of anequilateral triangle along a plane of each of the lower wafer, upperwafer and spacer wafer, respectively.
 8. The method of claim 1, furthercomprising the step of forming a plurality of optical element apertureswithin the spacer wafer respectively corresponding to the opposingplurality of first and second optical elements.
 9. The method of claim8, the spacer wafer including an optically opaque material chosen fromthe materials consisting of metal, plastic, ceramic and/or compositematerials, the optically opaque material being opaque to infrared (IR)wavelengths.
 10. The method of claim 4, the optical element apertures(i) allowing mechanical clearance between opposing ones of the first andsecond lenses of the lower and upper wafers, respectively, and/or (ii)allowing infrared and/or visible radiation to pass, without refractionor absorption, between opposing ones of the first and second opticalelements of the lower and upper wafers.
 11. The method of claim 9, theoptically opaque material being opaque to long wave infrared (LWIR)wavelengths.
 12. The method of claim 1, each of the plurality ofalignment elements being spherical.
 13. The method of claim 1, each ofthe plurality of alignment elements having a first thickness that is atleast as great as a second thickness of the spacer wafer.
 14. The methodof claim 1 further comprising the step of bonding, adhering, weldingand/or soldering one or more of the lower wafer, upper wafer, and spacerwafer to immobilize the wafers with respect to one another to form anarray of lens and spacer sets.
 15. The method of claim 14, furthercomprising the step of dicing the array of lens and spacer sets to formindividual lens and spacer sets.
 16. The method of claim 15, each of theindividual lens and spacer set including one of the first opticalelements, one of the second optical elements and a portion of the spacerwafer.
 17. The method of claim 16, the portion of the spacer waferincluding an optical element aperture.
 18. An alignment apparatuscomprising: a lower wafer having a plurality of first optical elementsand a plurality of first alignment features; an upper wafer having aplurality of second optical elements, each of the second opticalelements respectively opposing one of the plurality of first opticalelements, and the upper wafer further having a plurality of secondalignment features respectively corresponding to the first alignmentfeatures on the lower wafer; a spacer wafer, located between the lowerand upper wafers, having a plurality of alignment apertures completelytherethrough, each of the plurality of alignment apertures completelyenclosed by the spacer wafer; and, a plurality of alignment elements,each respectively located within individual ones of the plurality ofalignment apertures; wherein the alignment elements, the first alignmentfeatures, and the second alignment features cooperate to passively alignthe upper wafer to the lower wafer in at least four degrees of freedom,the four degrees of freedom including: X- and Y-coordinates definingcentering of each one of the plurality of first optical elements to arespective one of the plurality of second optical elements, aZ-coordinate defining spacing between the lower wafer and the upperwafer, and rotation of the lower wafer with respect to the upper wafer.19. The alignment apparatus of claim 18, characterized by one or more ofthe following: the lower wafer having a substantially planar uppersurface, the upper wafer having a substantially planar lower surface,and the spacer wafer having substantially planar counterfacing surfaces.20. The alignment apparatus of claim 18, the spacer wafer having aplurality of optical element apertures for at least one of: (i)mechanical clearance for one or more of the plurality of first andsecond optical elements, and (ii) allowing at least one of IR andvisible radiation to pass between opposing ones of the first and secondoptical elements without absorption.
 21. The alignment apparatus ofclaim 20, the optical element apertures being at least one of roundedrectangles, circles, and other shapes respectively matched to anoptically active area of the plurality of first and second opticalelements.
 22. The alignment apparatus of claim 20, the spacer waferincluding metal, plastic, ceramic or composite materials that areoptically opaque to IR radiation wavelengths.
 23. The alignmentapparatus of claim 18, the spacer wafer including a solid piece of IRtransparent material.
 24. The alignment apparatus of claim 18, whereineach of the plurality of alignment elements is in contact with both of acorresponding first alignment feature and second alignment feature whenthe lower wafer, spacer wafer and upper wafer are stacked on top of oneanother.
 25. The alignment apparatus of claim 18, wherein each of theplurality of alignment elements has a maximum width that is smaller thana respective second minimum width of the plurality of alignmentapertures.
 26. The alignment apparatus of claim 18, the plurality offirst and second alignment features including at least one of conicalindentations, spherical indentations, oval features, oblong features,grooves, radial grooves, and semispherical depressions within the lowerand/or upper wafers, respectively.
 27. The alignment apparatus of claim18, the plurality of first and second alignment features and theplurality of alignment apertures being spaced near the periphery of thelower wafer, upper wafer and spacer wafer, respectively.
 28. Thealignment apparatus of claim 18, the plurality of first and secondalignment features and the plurality of alignment apertures eachcomprising three first and second alignment features and three alignmentapertures, respectively, the three first and second alignment featuresand three alignment apertures being spaced apart to form points of avirtual an equilateral triangle in planes of each of the lower, upperand spacer wafers, respectively.
 29. The alignment apparatus of claim18, each of the plurality of alignment elements having a first thicknessthat is at least as great as a second thickness of the spacer wafer. 30.The alignment apparatus of claim 18, further comprising at least one ofan adhesive, bonding material, weld, and solder for immobilizing thelower, spacer and upper wafers with respect to one another.
 31. Thealignment apparatus of claim 18, the plurality of first and secondalignment features including different ones of alignment features chosenfrom the group including conical indentations, spherical indentations,oval features, oblong features, grooves, radial grooves, andsemispherical depressions.
 32. An apparatus for forming an athermalizedinfrared imaging system, the apparatus comprising: a lower wafer havinga plurality of first optical elements and a plurality of first alignmentfeatures; an upper wafer having a plurality of second optical elements,each of the second optical elements respectively opposing one of theplurality of second optical elements, each of the optical elementsrespectively opposing one of the plurality of first optical elements,and the upper wafer further having a plurality of second alignmentfeatures respectively corresponding to the first alignment features onthe lower wafer; whereby the lower and upper wafers are passivelyaligned via alignment elements located within alignment apertures of aspacer wafer, and the lower, upper and spacer wafers are diced such thatthe infrared optical imaging system formed therefrom is athermalized;wherein the first alignment features, and the respective first andsecond alignment features cooperate to passively align the upper waferto the lower wafer in at least four degrees of freedom, the four degreesof freedom including: X- and Y-coordinates defining centering of eachone of the plurality of first optical elements to a respective one ofthe plurality of second optical elements, a Z-coordinate definingspacing between the lower wafer and the upper wafer, and rotation of thelower wafer with respect to the upper wafer.